The present invention generally relates to receivers and transmitters for radio frequency (RF) signals, and particularly relates to direct conversion receivers.
Unlike a superheterodyne receiver, which converts an input RF signal to an intermediate frequency, a direct conversion receiver does not convert the input RF signal to an intermediate frequency. A direct conversion receiver mixes directly to a direct current (DC) signal, and is sometimes referred to as a zero IF receiver because the intermediate frequency is zero hertz (DC). The modulation information only is represented in the down conversion, and there is no carrier information that is typically associated with an intermediate frequency. In a direct conversion receiver the local oscillator signal is operating at the same frequency as the input RF signal. U.S. Pat. Nos. 5,438,692 and 5,548,068 disclose conventional direct conversion receivers.
In direct conversion, the modulation information is preserved through quadrature down conversion, which involves mixing the incoming line or carrier with a local oscillator signal along two different paths. The local oscillator signal along one path may be at zero phase (0xc2x0) with respect to the input RF signal, and may be phase shifted to 90xc2x0 along the other path. Alternatively, one path may be at xe2x88x9245xc2x0 while the other is at +45xc2x0 with respect to the input signal. See for example, U.S. Pat. No. 5,303,417. In any event, the circuit paths are typically mutually 90xc2x0 different in phase, and one path is referred to as the I channel while the other is referred to as the Q channel. The quadrature down conversion method preserves the necessary phase information within the input signal.
As shown in FIG. 1, a direct conversion circuit of the prior art includes an input port 10 for receiving an input RF signal from an RF antenna. The input signal is divided between two input paths. The input signal in one input path is mixed at mixer 12 with the local oscillator signal 14 at zero degrees phase shift. The input signal in the other input path is mixed at mixer 16 with the local oscillator signal at 90xc2x0 phase shift. The 90xc2x0 phase shift in the local oscillator signal is achieved by phase shift device 20. The local oscillator signal is produced in the illustrated conventional local oscillator circuit by first mixing two signals from voltage controlled oscillators (VCOs) 22 and 24 at mixer 26. The VCOs 22 and 24 may or may not be integrated in the same integrated circuit (IC) as the mixers 12 and 16. The signal is then filtered at bandpass filter 28 to produce the local oscillator signal 14. The bandpass filter 28 is typically not integrated on the same IC as indicated by the dashed line 29. The frequency of the local oscillator signal 14 is the same as the frequency of the input RF signal at input port 10.
Interference may occur if the local oscillator signal radiates to the input RF signal. Because the frequencies of these signals are the same, the local oscillator signal cannot be frequency filtered from the incoming signal. The incoming signal would, in effect, be blocked. U.S. Pat. Nos. 4,811,425 and 5,428,837 are directed to reducing the effects of leakage of local oscillator signals to RF input signals in zero IF receivers.
Moreover, interference may occur if the RF input signal radiates to the VCO. Since VCOs are typically very sensitive, any signal that is close in frequency to the frequency of the VCO may interact with it, even if the signal comprises only a small amount of energy. This is because the VCO will selectively amplify signals at or near its frequency, causing a low energy noise signal at or near its frequency to appear to be a spectrally clean signal.
One way of overcoming this problem is to employ a VCO that operates at a frequency different than the input RF signal. The frequency of the VCO signal is then modified to produce a local oscillator signal at the same frequency as the input RF signal. For example, as shown in FIG. 1, the signal from one VCO 22 (at frequency F1) may be combined with the signal from another VCO 24 (at frequency F2) by mixer 26. The combined signal may then be filtered by a bandpass filter 28 to produce a local oscillator signal 14. The product, however, of the F1 and F2 signals, will include spurious signals that must be filtered out to produce the local oscillator signal. For example, the product of two sine functions sin(xcex1)xc3x97sin(xcex2) equals xc2xd cos(xcex1xe2x88x92xcex2)xe2x88x92xc2xd cos(xcex1+xcex2). Two frequencies would be produced at the mixer (F1+F2 and F1xe2x88x92F2), and one would have to be filtered out. It is typically necessary to do this type of filtering off IC, which further invites interference or leakage of the local oscillator signal to the input RF signal.
In other conventional local oscillator circuits, one VCO only might be employed and the output of the VCO would be input to a frequency doubler, then to a bandpass filter, and finally to the phase shift device 20. The frequency of the VCO (F1) could be one half the frequency of the RF input signal, and the frequency of the local oscillator would then be 2F1. In further conventional local oscillator circuits, the frequency of the VCO (F1) could be twice the frequency of the RF input signal, and the frequency of the local oscillator signal may be equal to xc2xdF1. This could also be achieved with one VCO (F1), whose output could be input to a one-half frequency divider to produce the local oscillator signal for input to the phase shift device 20. In each such circuit however, the local oscillator signal may still radiate to the RF input signal, and the VCO may be sensitive to harmonic frequencies of the RF input signal.
Such conventional techniques do not fully alleviate the interference problems. It is an object of the present invention to provide a direct conversion receiver or transmitter that has reduced leakage or interference between the radio frequency input signal and the local oscillator.
A direct conversion circuit for radio frequency signals is disclosed. The circuit includes a pair of quadrature related mixers, a phase shift unit, and a local oscillator. The pair of quadrature related mixers is coupled to a radio frequency signal input port for mixing down a radio frequency input signal. The phase shift unit is in communication with at least one of the pair of mixers for phase shifting a local oscillator signal. The local oscillator produces the local oscillator signal. The local oscillator includes a non-integer frequency multiplier for multiplying the frequency of a first voltage controlled oscillator signal by a non-integer value to produce the local oscillator signal.